An information search system is a system capable of providing a user with information retrieval service. Taking the search engine which is widely used on the Internet as an example, the search engine, as a search system applied in the field of Internet, has become an indispensable assistance tool for users accessing the web. From a perspective of the user, the search engine typically provides a webpage containing a search box, and the user inputs in the search box a keyword or another search condition. The keyword is submitted through a browser to the search engine, which in turn returns information that match the contents of the keyword input by the user.
In response to the same search request of the user (e.g., a search keyword input by the user searching for information), the search engine typically retrieves information items that match the request, the number of which may be up to tens to tens of thousands. From the perspective of the user, however, his attention is usually focused on the information highly ranked among the search results. Thus, how to rank search information that is particularly important directly influences the user's experience with the search engine.